Can compaction modeling and pore pressure prediction be simplified by the definition of effective stress?

Abstract

Disequilibrium compaction is a significant source of overpressures in sedimentary basins, and for decades relationships between effective stress and compaction state have been used in such settings to model compaction and to predict pore pressure. These relationships have proved to be robust when calibrated and applied in formations with similar geologic history, lithologies, and pressure controls. However, they are also found to break down in many cases and these failures often lead workers to invoke a variety of complicating processes which may or may not be supported by geologic analyses; in other cases, to match data, workers may simply invoke ad hoc piecewise adjustments. A consistent thread through almost all such studies is the definition of effective stress as the difference between total stress and pore pressure, as defined by Terzaghi. Given persistent cases where typical models break down, though, it may be fair to ask if we have been using the best approach. An alternative definition of effective stress proposed by Biot is investigated here with a large field dataset and shown to give simpler compaction trends which are also much more consistent from region to region than those obtained with a Terzaghi definition. While the processes often invoked to explain departures from Terzaghi effective stress trends are likely present in many areas, this work suggests that their prevalence and significance may be less than often thought, and that a simple change of effective stress definition may provide substantially improved models with less need to invoke other geologic explanations.